Scale / Definition of Engineering Scale

Drawings of small objects can be prepared of the same size as the objects they represent, A 150 mm long pencil may be shown by a drawing of 150 mm length. Drawing drawn of the same size as the objects, are called full size drawings. The ordinary full size scales are used for such drawings. A scale is defined as the ratio of the linear dimensions of element of the object as represented in a drawing to the actual dimensions of the same element of the object itself.
The scales generally used for general engineering drawings are.....
1: reducing scales
2: enlarging scales
3: full size scales.

The scale is indicated on the drawing at a suitable place near the title. The complete designation of a scale consists of word scale followed by the ratio, I. E. Scale 1:1 or scale, full size.
It may not be always possible to prepare full size drawings. They are, therefore, drawn proportionaly smaller or larger. When drawings are drawn smaller than the actual size of the objects (as in case of buildings, bridges, large machines etc.) the scale used is said to be a reducing scale (1:5). Drawings of small machine parts, mathematical instruments, watches etc. Are made larger than their real size. These are said to be drawn on an enlarging scale (5:1).
1. Engineer's scale: in the case, the relation between the dimension on the drawing and the actual dimension of the object is mentioned numerically in the style as 10 mm = 5m etc.
2: Graphical scale: the scale is drawn on the drawing itself. As the drawing becomes old, the engineer's scale may shrink and may not give accurate results.
However, such is not the case with graphical scale because if the drawing shrinks, the scale will also shrink. Hence, the graphical scale is commonly used in survey maps.
3: Representative fraction: the ratio of the length of the object represented on drawing to the actual length of the object represented is called the representative fraction